In recent years, it has become a desideratum to provide a high strength non-metallic fastener. Such fastener would provide significant advantages in that plastic components which are secured with metallic fasteners may tend to deteriorate due to galvanic corrosion and variations in coefficients of thermal expansion. Moreover, the use of metallic fasteners in aircraft and other such applications may produce a deviation in navigational equipment and problems with electronic equipment.
However, a non-metallic fasteners presently known have generally lacked the strength exhibited by metallic fasteners of comparative size, and this low strength is very apparent both in thread or threadlike areas. When axial forces are exerted on matingly threaded members, e.g., a nut and a bolt, the threads of the members are subjected to shear forces which may distort or otherwise damage the thread. Similarly, when axial forces are applied to the enlarged head portion of a bolt, the axial force required to sufficiently secure the fastened members may weaken the fastener at the load bearing flange which extends laterally beyond the fastener shaft.
To withstand such shear forces, threaded plastic members have included resin impregnated fibers. For example, glass fibers have been helically wound about the longitudinal axis of the fastener, i.e., longitudinally of the threads. In this instance, the shear forces on the threads have been opposed primarily by the resin bonding between the fibers. As the shear strength of the resin is generally less than that of the resin-impregnated fiber the strength of the members has been less than adequate, due primarily to the two-dimensional fiber orientation which is inevitably subjected to delamination.
Other methods which have been employed to increase the shear strength of threads include the use of mat reinforcement around pulltruded unidirectional fiber rod. The mat reinforcement has random fiber orientation with multi-directional properties, but as the fibers are not continuous the tendency toward delamination and relatively low strength persists.
U.S. Pat. No. 2,510,693 describes fastening members having a longitudinal fibrous reinforcing medium extending along the stem portion and into the head of the fastener. U.S. Pat. No. 2,685,813 describes a glass-fiber rivet body, including a spirally wound longitudinal fibers and, in an alternative embodiment, the longitudinal fibers are surrounded by braided threads in an essentially helical form.
U.S. Pat. Nos. 2,928,764 and 3,283,050 disclose methods and apparatus for the production of threaded fiber fasteners employing circumferential or helical fiber to form threads. U.S. Pat. Nos. 2,943,967 and 4,063,838 also detail the formation of threaded members with a combination of longitudinal and helical fibers or filaments.
U.S. Pat. No. 3,995,092 details the use of a plurality of laminated sections which are glued together to form the fastener, with the laminations substantially perpendicular to the surface of the threaded shaft. U.S. Pat. No. 3,495,494 again details the use of longitudinal but spirally wound fibers which are oriented along a generally serpentine path to conform substantially to the course of the threads.
According to the present invention, fastener members are formed from multi-dimensional woven fiber preforms having uniform isotropic properties, e.g., three-dimensional orthogonal blocks, molded with organic resins. The fasteners and the threads thereon thus contain a plurality of fibers disposed continuously through the fastener. Specifically, the fibers are disposed in a plane which is essentially perpendicular to the longitudinal axis of the fastener, i.e., as chords of the cross section of the fastener, with the end portions of such chordal fibers extending into the threads and other angular load-bearing flanges of the fastener. For example, the fibers are disposed at right angles in the lateral cross-sectional plane of the fastener so that the end portions of the fibers form the peaks and valleys of the threads. Such fasteners increase shear strength and resist delamination as a result of the continuous three-dimensional fiber placement. Moreover, the use of preformed, impregnated blocks allows the economical and advantageous production of fastener members without the need for complex winding or casting machinery or techniques.